Verified answer

well if f(x)=ax^n then f'(x)=anx^(n+1)

so here f'(x)=(4/5)x^(9/5)-(2/7)x^(8/7)

hence

f'(6)=(4/5)6^(9/5)-(2/7)6^(8/7)

=17.91186386 (8dp)

and

f'(1)=(4/5)1^(9/5)-(2/7)1^(8/7)

=18/35

=0.5142857143 (10dp)

f'(x) = 4/5x^(-1/5)-2/7x^(-6/7)

f'(6) = 0.49

f'(1) = 0.51